Electric musical instrument reverberation nonlinear control system



Aug. 26, 1969 H. LAUBE 3,463,868

ELECTRIC MUSICAL INSTRUMENT REVERBERATION NONLINEAR CONTROL SYSTEM Filed June 2, 1966 2 Sheets-Sheet l I l 6 &5 20 22 J2me 20/1 Aug. 26, 1969 H. LAUBE 3,463,868

ELECTRIC MUSICAL INSTRUMENT REVERBERATION NONLINEAR CONTROL SYSTEM Filed June 2, 1966 2 Sheets-Sheet 2 REVERE SPF/N6 TUNED C/FCl/l T5 United States Patent 3,463,868 ELECTRIC MUSICAL INSTRUMENT REVERBERA- TION NONLINEAR CONTROL SYSTEM Hans Laube, Chicago, Ill., assignor to Hammond Corporation, a corporation of Delaware Filed June 2, 1966, Ser. No. 554,727 Int. Cl. G10h N02 US. Cl. 841.24 5 Claims ABSTRACT OF THE DISCLOSURE A control system for proportioning an artificial reverberation signal relative to the main signal of an electrical musical instrument in which a nonlinear element is connected to receive the main signal and develops a nonlinear control signal thereacross which is fed back in inverse phase to reduce the gain of the reverberation signal nonlinearly as the level of the main signal increases.

This invention relates generally to acoustic systems and apparatus and more particularly to an artificial reverberation control system. Primarily, it is intended for organ use, but it may also be used to advantage with music reproducing systems such as record players and tape recorders, for instance.

In general, the invention contemplates controlling the relative amount of artificial reverberative effect dependent upon the loudness of the sound being produced. It has been generally recognized (see patents referred to hereinafter) that as the volume level of the music increases, the reverberation effect should not increase proportionately. More recently it has been discovered that for optimum regulation of this phenomenon it is necessary to provide a regulatory system which permits of considerable nonlinearity in its control function. The purpose of this invention is not to dictate just what nonlinearity curves should be followed, but to provide a system which frees the designer to introduce the nonlinearities which seem best to him under the circumstances. The nonlinearities contemplated include not only general signal level, but also frequency discrimination and the like as will appear.

This invention constitutes an improvement over the reverberation systems disclosed by Hammond Patent No. 2,949,805 and Hanert Patent No. 3,070,659, in which the output to a reverberation speaker does not rise as much as the output of the main speaker when the total signal increases in amplitude. Similarly, in Meinema Patent No. 3,037,414 there is disclosed an artificial reverberation control apparatus in which a nonlinearly attenuated signal derived from the direct output speaker is used to drive a reverberation amplifier. In the present system, as in these other systems, the output of the reverberation amplifier increases less than does the output to the direct speaker when the total signal level rises, but much greater and more precise control over the ultimate result is aliorded by the present invention.

In this invention the amount of reverberation is automatically related to loudness of the signal in the reverberation channel. For any desired volume or intensity level the amount of reverberation in the total sound is automatically set at an optimum without effort on the part of the organist who is producing the music. For loud audio signals the proportion of reverberation in the total signal is automatically reduced; for soft signals the intensity of 3,463,868 Patented Aug. 26, 1969 the reverberating component relative to the non-reverberating component is very considerably increased, and the degree of nonlinearity of this effect is easily controllable by the designer, and, as will be seen presently, other nonlinearities can easily be introduced into the system if desired.

In general, the desirable result is accomplished by a feedback system in which a nonlinear resistor with a positive resistance coeflicient is placed in the input to a reverberation unit, and the signal voltage developed across this resistance is returned to the input of a reverberation amplifier on an inverse feedback relationship. Other voltage dividers or nonlinear systems are used in the circuit as required, but the important thing is that the amount of the inverse feedback signal voltage is nonlinear with respect to the amplifier output so that the gain of the reverberation amplifier is progressively decreased as the signal increases.

One of the objects of the present invention is to provide an improved means for controlling the amount of reverberation so that it is substantially optimum for any desired signal or sound intensity level.

A further object of the present invention is to provide an improved reverberation control system wherein a feedback signal is developed across a nonlinear element and used to inversely control the intensity of a reverberation output signal.

A further object of this invention is to provide a simple and inexpensive apparatus for automatically accomplishing reverberation level control.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a chart illustrating generally the objects and results achieved by the invention;

FIG. 2 is a partial schematic and block diagram illustrating the invention;

FIG. 3 is a partial schematic and block diagram showing a modification of the invention; and

FIGS. 3a, 3b, and 3c show modifications which can be incorporated into the circuits of FIGS. 2 or 3.

Referring to FIG. 1, there is shown on a logarithmic scale the desired output in watts from the direct speaker and the reverberation speaker in terms of the position of the swell pedal of an electric organ or output control of some other electrical musical signal source. Curve A represents the direct signal and is approximately a straight line, while the curve B represents the reverberative portion of the signal. These curves show that at low wattage output the intensity of the reverberative component is about the same as the direct signal and rises generally along with an intensity rise of the direct component, but that at the loud end of the scale a change in the intensity of the main signal is accompanied by only a small change in the level of the reverberative component. In a particular practical system, for instance, at the maximum open position of the swell pedal the intensity of the reverberative component of the sound is approximately 8 to 12 db (decibels) lower than that of the direct signal.

In FIG. 2 the source of the sound in the form of an electrical signal is represented by a block 10. This source may be an electric organ generating system, a phonograph, a pickup of music recorded on magnetic tape, or other similar means. The output terminals 11 and 12 are respectively connected to an output conductor 14 and ground. Also connected to terminal 11 is a conductor 16 which leads to a direct signal preamplifier 18, a direct signal power amplifier 20 supplied by the direct signal preamplifier and a direct signal speaker 22 supplied by the power amplifier.

Conductor 14 leads through resistor R1 to the input terminal 38 of a reverberation driver amplifier 40. The primary of an impedance matching transformer.42 is connected from the output of the reverberation driver amplifier 40 to ground. The secondary of the trans former 42 is connected at one side to ground through the driver element of a reverberation spring unit 44, whereas the other side of the secondary, represented by terminal 52, is connected to ground through a reverberation drive current sensing resistor R3.

Connected in series with the output of the reverberation unit 44 are, in order, a reverberation recovery amplifier 46, a reverberation power amplifier 48, and a reverberation speaker 50. A reverberation unit of the type contemplated at 44 forms the subject matter of Meinema et al. Patent No. 2,982,819. A feedback resistor R2 is connected between transformer terminal 52 and the input terminal 38 of the reverberation driver amplifier 40. Resistors R1 and R2 act as negative feedback voltage dividers, and adjustment of their values will determine the effectiveness of the fed back voltage in reducing the gain. The reverberation drive current sensing resistor R3 is a nonlinear device, such that its resistance increases with increased current in the transformer secondary circuit. For this purpose it is preferred to use an inexpensive small light bulb of the type suggested in the previously referred to Patent No. 3,037,414. As the current increases, the light bulb glows more brightly and its resistance increases, since such incandescent lights have a positive temperature coetficient of resistance. Thus, an increase in signal current increases the resistance at R3 and increases nonlinearly the feedback signal through R2 to the input of amplifier 40. Since the fed back signal is phased oppositely with respect to the signal from source 10, the gain of amplifier 40 is reduced with an increase in the signal level. By proper selection of the negative feedback voltage divider resistors R1 and R2, and the characteristics of R3, and the basic gain of amplifier 40, just about any desired shape of the function output current versus input signal level can be achieved. In the particular embodiment shown, the output current versus input signal level is adjusted to achieve approximately the response shown in FIG. 1.

FIG. 3 is a modification of the invention shown in FIG. 2. Here, there is a single output speaker 28 as opposed to the two channel output shown in FIG. 2. The tone signal input, which has a variable intensity, is supplied from block 10, and terminals 11 and 12, as in FIG. 2. These terminals are respectively connected to a connector 14 and to ground. Connector 14 is connected through a resistor R1 to the input 15 of a reverberation driver amplifier 40, the output of which is connected through the primary of transformer 42 to ground through a small light bulb which acts as a nonlinear series resistor R3. A terminal 60 between the transformer primary and resistor R3 is connected through resistor R4 back to the amplifier input at 15.

The secondary of transformer 42 is connected to drive the reverberation spring unit 44. The input of a reverberation recovery amplifier 46 is connected to the output of the reverberation spring unit 44, and the output therefrom is connected to a linear mixer 24 which combines the signals from the direct outnut preamplifier 18 and the reverberation recovery amplifier 46. The output from linear mixer 24 is connected to a power amplifier 26 which in turn is connected to an output speaker 28. The operation of the embodiment shown in FIG. 3 is similar to that of FIG. 2 in that the current sensitive nonlinear resistor R3 feeds back a nonlinear out of phase voltage which increases nonlinearly with an increase in signal from the source 10, thus reducing the gain of amplifier 40 upon an increase in signal amplitude.

Note that with either of these circuits the nonlinear resistor R3 has a dual effect upon the character of the curve B of FIG. 1. First, an increase in signal current increases the resistance of R3, and this has a direct eifect upon the series connected reverberation unit; that is, the resulting increase in impedance of the reverberation unit input circuit will reduce the current therethrough and have a signal compressing effect. This is the sole effect taught in previously mentioned Meinema Patent No. 3,037,414. The second effect is cumulative and is the result of inverse feedback of the signal developed across nonlinear resistor R3 to the'amplifier input. An important advantage of the present circuit is not only that it increases the compression effect as compared with Meinema, but that the feedback path lends itself well to additional control, such as that established by the resistors R1 and R4, which can be adjusted to change curve B of FIG. 1.

The feedback path can also be used to introduce additional nonlinearities if desired. As an example, if it is wished to compress the high frequencies more than the low frequencies upon a signal increase, a. capacitor 62 (FIG. 3a), of appropriate value, can be connected in series with R4 so as to remove some of the low frequencies from the inverse signal fed back to the amplifier input. A choke 64 (FIG. 3b), connected from terminal 52 or 60 to ground, would accomplish much the same effect. Other arrangements for obtaining special effects will suggest themselves and may make various uses of capacitors, chokes, resistors, and/or tuned circuits 66 (FIG. 30). The important consideration is that the basic circuit provides a system which facilitates the introduction of various nonlinear elements to produce special effects.

Summarizing, the invention contemplates using a nonlinear resistor with a positive resistance coefficient or its equivalent in the drive line to the reverberation unit, and feeding back the voltage developed across the nonlinear element to the input of the reverberation amplifier on an inverse phase relationship. Thus the amount of the inverse feedback voltage is nonlinear, whereby the gain of the reverberation amplifier is decreased as the signal volume rises.

In addition, it is contemplated to provide frequency discrimination by means of capacitors, inductors, and/or resonlant circuits connected so as to affect the fed back s1gna.

Various modifications may be made in the invention w1thout departing from the spirit and scope thereof, and 1t is desired, therefore, that only such limitations shall be placed thereon as are imposed by the prior art and are set forth in the appended claims.

I claim:

1. A reverberation control system comprising a source of musical tone signals of changeable amplitude, means to transduce said signals, an amplifier having an input connected to receive said source signals, a reverberation apparatus, means coupling the output of said amplifier to the reverberation apparatus input, means to transduce the output of said reverberation apparatus, said coupling means including a nonlinear impedance element, means inversely to feed back the nonlinear signal developed across said nonlinear element to the input of said amplifier to reduce the effective gain of said amplifier nonlinearly as the amplitude of said source signal increases.

2. In a reverberation control system, the combination recited in claim 1 wherein said nonlinear element is a current sensitive resistor.

3. In a reverberation control system, the combination recited in claim 1 wherein said inverse feedback circuit includes means for introducing an additional nonlinearity into the inverse signal fed back to said amplifier input.

4. In a reverberation control system, the combination recited in claim 4 wherein said inverse feedback circuit 5 6 includes reactive means to discriminate the inverse feed- 3,037,414 6/1962 Meinema 841.24 back signal on a frequency basis. 3,070,659 12/ 1962 Hanert 84-124 X 5. In a reverberation control system, the combination 3,267,197 7/1966 Hurvitz 84-1.24

recited in claim 1 including, means providing a mixer circuit connected to combine the output signals from said 5 HERMAN K. SAALBACH, Primary Examiner iource wlth the output signals from said reverberation s CHATMON, Assistant Examiner pparatus.

References Cited UNITED STATES PATENTS 1 27 US. Cl. X.R. 

